Interpreting the Tree Diagram or List of Subgroups on a Tree of Life Page

Most Tree of Life branch pages feature a tree diagram that shows the relationships
between the subgroups of the group that the page is about. The root of
this tree connects the subgroups to their containing group and the rest
of the Tree of Life. The basal branching point in the tree represents
the ancestor of the other groups in the tree. This ancestor diversified
over time into several descendent subgroups, which are represented as
internal nodes and terminal taxa to the right.

For example, consider the following tree:

This phylogeny shows only a few of the many millions of living species,
and it is therefore far from complete, but it is useful for this example.
This phylogeny states that there was an ancestral species that split to
give rise (billions of years later) on the one hand to bacteria, and on
the other to multi-celled organisms we see today. The multi-celled lineage
split again, and again, diversifying into numerous forms. The phylogeny
states that there was an ancestral species that gave rise to mammals and
birds, but not to the other species shown in the tree (that is, mammals
and birds share a common ancestor that they do not share with other species
on the tree), that all animals are descended from an ancestor not shared
with fungi, plants, and bacteria, and so on.

Grey tree diagrams

Some Tree of Life pages have their trees printed in a lighter color. Such
lighter trees characterize temporary or skeleton pages which generally
have little content. They have been created to serve as links connecting
more complete pages. grey trees should be interpreted with caution, because
they are usually taken from the literature with little critical evaluation
by Tree of Life authors, editors, or peer-reviewers.

Polytomies

The phylogeny shown above is dichotomous, in that at all
branch points there are two immediate descendents. If there are more than
two immediate descendents at a given branch, then the tree is polytomous.
For example, in the following tree, there is a polytomy from which dragonflies,
mayflies, and beetles arise, indicating that the relationships between
these three lineages is not yet clear:

We don't know if dragonflies and mayflies are more closely related to each
other than either is to beetles, or whether beetles are more closely related
to mayflies, or beetles to dragonflies.

Groups that may not be monophyletic

Branches that are a represented by a hatched line rather than one solid
bar indicate that the monophyly of the group may be uncertain. In the
following tree, group A may not be monophyletic, indicating that one of
Group A's subgroups may be more closely related to group B than to other
subgroups of Group A.

Groups that are not monophyletic

Branches that are a group of three lines rather than one solid bar indicate
that the group is not monophyletic. Generally, we try to avoid non-monophyletic
groups in the Tree of Life. However, we sometimes need to work with them,
if we want to divide the diversity of a large group into manageable pieces
in the absence of phylogenetic hypotheses. In the following tree, group
A is non-monophyletic, indicating that Group B's closest relative is actually
a subgroup of Group A, but we don't know which subgroup that is.

Groups of uncertain phylogenetic position

Branches that begin with a question mark indicate that the phylogenetic
position of a group is uncertain (incertae sedis). In the following tree,
Group C is thought to be the sister group of Group D, but this hypothesis
is considered to be preliminary. Group E is thought to belong to the group
treated on this page, but there are currently no good hypotheses for the
subgroup's position within the group.

Groups that are extinct

If a group does not have any living representatives, i.e., members of the
group are known only from the fossil record, the name of the group is
followed by an extinct icon .
In the following tree, Group A is entirely extinct; while both Group B
and Group C have at least some subgroups that are alive today.

Labelled branches

On some ToL pages, the tree may have one or more labelled branches. The
name printed on the branch is that of the group associated with the node
(branching point) at the end (i.e., to the right) of the branch. In the
following tree, Group B is the sister group of Group A, and it contains
both Group C and Group D.

On the Tree of Life, branch labels are used to indicate group names that
are noteworthy, eventhough that particular group does not have its own
Tree of Life page.

Lists of subgroups

If the phylogenetic relationships among the subgroups of a group are unknown,
the Tree of Life page will show list of subgroups instead of a tree. On
most ToL pages, this is just a plain list of group names, but some authors
provide additional information in the form of an indented list. In the
following list, the cascading indentations indicate the names of groups
of decreasing inclusiveness: The group treated on this ToL page has three
immediate subgroups: Group A, B and E. The subgroups of Group A will be
treated on a separate ToL page, but the subgroups of the other two groups
are listed on the current page: Group B has two subgroups, Group C and
D. Group E has three subgroups, Group F, J, and M.

Group A

Group B

Group C

Group D

Group E

Group F

Group J

Group M

If there are any non-monophyletic groups or groups of uncertain phylogenetic
position (incertae sedis) included in a classification, these groups are
labelled with parenthetical remarks: